Easy DIY CPU

Carl Burch's Logisim may be compact, totally portable, and free, but that doesn't stop it from being one of the most useful pieces of educational software we've seen yet. It lets you map out simulated logic circuits such as CPUs using preconfigured elements such as AND, OR, and NOT gates; plexers; memory; and wiring, including a tool for drawing color-coded wiring harnesses that simplify programming and debugging. Logisim is simple enough to use as an educational tool for learning the basics of logic circuits, but it can also simulate sophisticated processing pathways. You can use it to create simple circuits and use them to build more complex circuits.

As a portable application, Logisim runs as soon as you click on its program file, so you can store the program on a portable drive or storage medium and run it on any Windows PC without having to install it. Logisim's compact, efficient interface is dominated by the pegboard main view. The toolbar is filled with icons that represent logical operators such as AND or NOT; tools for adding pins and other components; and tools for adding text, shapes, and such. A small tree view offers folders for Wiring, Gates, Plexers, Arithmetic, Memory, Input/Output, and Base. Selecting any tool or object displays its name and properties in a small grid beneath the tree view. There is a very simple, intuitive setup that involves starting a project, dragging and dropping the various elements of your circuit onto the board, and then drawing connecting wires from point to point. Logisim has all kinds of tools to help, such as built-in libraries and simulation options. A basic lack of expertise did not stop us from drawing up and saving some very basic circuits.

Logisim is free and portable, and runs on just about any Windows machine from 98 on, making it almost universally accessible--another major plus for it as an educational tool, since a textbook you can't take home makes it awfully hard to do homework! Putting Logisim in the hands of bright youngsters today could lead to a brighter tomorrow.